Error messages will be available in the field on which the error
occurred. You can access error_fields on the form or on Compound
fields (and subclasses, like Repeatable).

The process method of this field runs the process methods on the child fields
and then builds a hash of these fields values. This hash is available for
further processing by actions in HTML::FormHandler::Field and the validate method.

You can set defaults or other settings in a build_update_subfields method,
which contains attribute settings that will be merged with field definitions
when the fields are built. Use the by_flag key with repeatable, compound,
and contains subkeys, or use the all key for settings which apply to all
subfields in the compound field.